Wallclock time in logs

Wallclock time in each line of the logs is useful in at least the following situations:

• when checking an active long-running build, to see how long it has been running
• when checking an active long-running build, to see when the last log line was output (has it hung, maybe?)
• when reviewing a historic log, to establish at what time a specific event happened (e.g. to help troubleshoot an outage, or performance problem in ci infrastructure)

This is not to say that wall clock time is enough. We also need elapsed time for steps, so that we can assess where bottlenecks happen and where to focus attention for performance improvements.

FWIW YBD logs both wallclock and elapsed, at the expense of long log lines, e.g.

15-09-13 00:05:09 [4/698/698] [stage2-glibc] Elapsed time for assembly of stage2-glibc.d9a5b38ff5115592af4ad878a5eda98915582e3e16c9d11b5176e123058bcf67 00:01:30